1. Field of the Invention
The present invention relates to an optical information recording medium, and in particular to an optical information recording medium on which information is recorded and with which information is reproduced by irradiating the medium with laser light from the side of the medium at which a thin protective layer is disposed.
2. Description of the Related Art
A CD-R is a writable, optical information recording medium (optical disc) on which information can be written only once by irradiation with laser light, and is widely known. A CD-R typically comprises a transparent disc substrate having successively disposed thereon a dye recording layer including an organic dye, a light-reflective layer including metal such as gold, and a protective layer made of resin. Information is recorded on the optical disc by irradiating the optical disc with near infrared laser light (i.e., laser light generally having a wavelength of around 780 nm). Specifically, the portion of the dye recording layer that is irradiated absorbs light, whereby temperature increases at the irradiated portion. The increase in temperature produces a physical or chemical change (e.g., formation of pits) to alter the optical properties of the irradiated portion, whereby information is recorded. The information thus recorded on the optical disc is ordinarily reproduced by irradiating the optical disc with laser light having the same wavelength as that of the light used to record the information and detecting a difference in reflectance between the region of the dye recording layer whose optical properties have been changed (recorded portion) and the region of the dye recording layer whose optical properties have not been changed (unrecorded portion).
DVD-Rs, which are writable optical discs that can be recorded at a higher density than CD-Rs, have also come into widespread use and continue to gain prominence as media on which massive amounts of information can be recorded. In order to increase the recording density of optical information recording media such as CD-Rs and DVD-Rs, it is effective to reduce the spot diameter of laser beam. The spot diameter is proportional to λ/NA (where NA is the numerical aperture of the lens and λ is the wavelength of the write laser). In order to reduce the spot diameter, it is effective to shorten the wavelength of the write laser or to enlarge the numerical aperture of the lens. When the numerical aperture NA of the lens is enlarged, spherical aberration thereof increases in proportion to the 4th power of the numerical aperture. Thus, it is necessary to reduce the thickness from the surface upon which light is made incident to the recording layer. A DVD-R achieves a recording capacity 7 times greater than that of a CD-R by enlarging the numerical aperture of the lens more than a CD-R (from 0.45 for a CD-R to 0.6 for a DVD-R) and by shortening the wavelength of the write laser (from 780 nm for a CD-R to 635 nm for a DVD-R). Moreover, by increasing the numerical aperture of the lens, substrate thickness at the side where light is made incident is reduced to 0.6 mm, which is half the thickness of a CD-R, and spherical aberration is lowered.
With the development of laser technology in recent years, short-wavelength lasers such as blue lasers have come to be used. Together with this development, there has been progress in the development of new optical information recording media capable of being recorded at high density by light having wavelengths shorter than conventional recording wavelengths (780 nm or 630 nm). As mentioned in the preceding paragraph, the smaller the spot diameter of laser beam is, the higher the recording density can become, and it is necessary to form a recording layer at a shallow position from the surface in order to reduce the spot diameter. To this end, Japanese Patent Application Laid-Open (JP-A) No. 2000-11454 proposes an optical information recording medium including a substrate having disposed at the opposite side thereof a thin protective layer of a predetermined thickness, with short-wavelength light being irradiated from the side of the medium disposed with the thin protective layer to record information on the medium. It has been customary to use photocurable resins, such as resins photocurable by ultraviolet light, to form the protective layer.
However, when a thin protective layer is disposed at the side of the medium opposite to the side at which the substrate is disposed, it becomes necessary to successively dispose on the substrate a light-reflective layer, a recording layer and the thin protective layer, whereby the recording layer is also irradiated with light at the same time that the photocurable resin is irradiated with and photocured by light to form the thin protective layer. There is thus the problem that the recording substance in the recording layer is partly deteriorated when the medium is fabricated, whereby recording properties are adversely affected. In particular, when the recording layer includes a dye, the dye absorbs light of wavelengths even shorter than 400 nm, including ultraviolet light, whereby the layer easily decomposes and is seriously deteriorated.
JP-A No. 11-27314 proposes an optical information recording medium comprising a substrate having adhered thereto a transparent film via a pressure-sensitive adhesive sheet or a dry polymer sheet, whereby a light-transmitting, thin protective layer is thus formed without irradiation with ultraviolet light. However, there is the problem that air bubbles are produced between the transparent film and the substrate when the transparent film is adhered to the substrate via the pressure-sensitive adhesive sheet, which results in poor adhesion.